In the days of buffer underrun protection (whatever drive "X" calls it), Simulation is highly overrated ... actually, Nero will not allow you to simulate unless underrun protection is OFF - this program may be applying similar logic).
Simulation finds only a very limited number of problems:
- Buffer underrun was the main one
- Problem reading a file, or decoding an audio file (caching works just as well for that)
- Tracking fault on the blank (well, if you MUST save it, you could use for a shorter burn) - I've had "pinholed" blanks a couple of times, and stingy sod that I am, I HAVE used them to whatever capacity I estimated to be good.
The only real use I've had for simulation, is in an overburn test.
In short, simulation tells you the burn will COMPLETE, but not that it will be GOOD.